Hmm... Well, as for getting them to work, I think you can copy and pste them (with Paint) into the slots where the ones being used now are.
As for the purpose, they may have been used in the development stages of the game. Then, although they chose a certain set of ones for the final design, they may have included those designs for those of us who wanted to "customize" our game--the same way they let us edit the rules, game, and city .txt files.